Electric motors including rotors comprising permanent magnets, such as brushless DC motors, commonly include a rotor core or backiron supporting a plurality of separately formed magnets. In a known construction of the rotors, the magnets are adhesively retained to the backiron in the axial, radial and circumferential directions, where the adhesive may be the only means of retaining the magnets to the backiron. The magnets may be surrounded by a magnet rotor cover that fits tightly over an outer surface of the magnets and that operates as a secondary means of retaining the magnets in the axial and radial directions, but does not prevent movement of the magnets in the circumferential direction. For example, the rotor cover may extend longitudinally over the magnets and be formed over the axial ends of the magnets to provide a radial and axial retention of the magnets. Additionally, an endcap may be located to cover the ends of the magnets to prevent damage to the material forming the magnets along the edges where the rotor cover is formed over the axial ends. However, the rotor cover and endcap do not provide a positive retention mechanism for retaining the magnets in a circumferential direction to prevent the magnets from rotating around the backiron in the event of failure of the adhesive.
The magnets may also be located at predetermined circumferential locations by various features. For example, the backiron may be formed with small tabs located between adjacent magnets and thus prevent rotation of the magnets around the backiron if the adhesive should fail. However, the use of such features affects the manufacturing process, the resulting motor performance, and the complexity of the backiron design. One or more of the features described above are disclosed in U.S. Pat. No. 5,563,463, which patent is incorporated herein by reference.